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The Ames test is a test used to determine whether or not genetic mutations, or changes, are caused by a material. A special type of salmonella bacteria is combined with animal liver extracts. This combination is then exposed so that the substance can be examined and tested for mutagenesis, or signs that the bacteria mutated. This biological assay is used to analyze a chemical compound's mutagenic potential.
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Several strains of the Salmonella typhimurium bacterium carrying the genes associated with the histidine synthesis, are used in this test, so that growth requires histidine. The ability of the mutagen in how it causes a growth reversion in a histidine-free medium, is the variable being tested. The tester strains are specifically created so that they have both point mutations and frameshift in the genes that are necessary to synthesize histidine, which then allows the mutagens to be detected acting through different mechanisms. Sometimes reversions are caused in just a strain or two when the compounds are quite specific. The genes that are responsible for causing lipopolysaccharide synthesis, are carried in the gene mutations of the tester strains, making the bacteria's cell wall more permeable, and the test is make more sensitive because this is in the excision repair system. To stimulate the effect of metabolism, rat liver extract is some added, because benzopyrene, and some other compounds, are not themselves mutagenic, but their metabolic products are.
The bacteria are spread on an agar plate containing a small amount of histidine. The small amount of histidine that is within the growth medium encourages the bacteria, for an initial time, to grow and have the chance to mutate. Only the bacteria that have experienced a mutation allowing them to gain the capability to create its own histidine will survive, when the histidine is depleted. For 48 hours, the plate is incubated. The number of colonies observed is proportional to the mutagenicity of a substance.
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Salmonella is not the best model for humans due to being a prokaryote. For eukaryotic cells, an in vitro model, that has been adapted, was made. An example is yeast structure. Hepatic system-formed metabolites are not counted for by the modified tests. To further any drug development, the metabolite responsible for a positive Ames test, further testing is necessary.
Drugs containing the nitrate moiety are indeed safe, but sometimes produce a positive Ames test. Nitroglycerin is a solid example of this. To disprove a positive test, long outcome and toxicology studies with such compounds are needed.
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John W. Kimball. (2006). Ames Test. Retrieved on September 1, 2010 from Kimball's Biology Pages: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AmesTest.html
San Diego State University. (2003). The Ames Test. Retrieved on September 1, 2010 from San Diego State University: http://www.sci.sdsu.edu/~smaloy/MicrobialGenetics/topics/rev-sup/ames.html